Statistics Definitions > Sequence Effects
What are Sequence Effects?
Sequence effects happen when sequences of conditions in an experiment interact with each other. In other words, earlier events can have an effect later events, or events that happen at the same time can affect each other. They can become confounding variables in an experiment. For example, a researcher may set a certain tone at an early stage in the experiment, affecting later responses.
Another example of a sequence effect can be found in Harry Helson’s study of sequence effects in perceptual judgments (1964). Helson and his students gave a series of weights to participants and found that a weight would be judged “heavy” or “light” depending not on the actual weight, but upon the other weights presented at the same time.
Order Effects vs. Sequence Effects
A similar effect to a sequence effect is an order effect, where the actual order of treatments influences the outcome. For example, let’s say you wanted to test how well people learned to solve a puzzle. The first attempt at a puzzle will almost always be poor, and as people warm up to the task, later efforts will be better. This is an order effect — the first attempt will likely be the worst merely by its placement in the list. On the other hand, sequence effects are interactions between the stages of the experiment. For example, let’s say puzzle #2 was, on a difficulty scale of 1 to 10, a 10. The next puzzle given was a medium difficulty puzzle (a 4 or 5), but it seems easier because the puzzle previous to it was much harder. In sum:
- Order effects result from the ordinal (numbered) position of a condition in an experiment.
- Sequence effects result from any interaction between the conditions.
References:
Helson, H. Adaptation-level theory. New York: Harper & Row, 1964.
McBurney, D. & White, T. (2009). Research Methods.